In drilling vessels, the dynamic positioning (DP) system has a great importance for the operation, since it ensures the station-keeping ability for the drilling operation. However, an emergency situation involves ungoverned drift due to problems associated with the DP system failures, such as thrusters, generators, powerbus, or control system. During this situation, the vessel drift is subjected to the influence of environmental conditions and the drift can lead to collisions with floating obstacles or submerged systems, wellhead emergency disconnection, damage to equipment and potentially causing major environmental disasters. It is then necessary to define a safety region for the drilling ship operation and to determine the limiting operation offset that the drilling vessel can disconnect from the wellhead without damage to any equipment. This offset limit is obtained through a riser analysis and drift-off study, important inputs for the Well Specific Operating Guidelines (WSOG). A validated time-domain simulator is required and able to predict the vessel drift trajectory after the DP failure under several environmental conditions. The aim of this work is to present a large set of model and full-scale drift tests and the validation of a time-domain numerical simulator (Dynasim), based on the main parameters of the drift tests: drift distance, heading variation, and trajectory. The comparisons between the numerical simulation results with full- and model-scale data demonstrated the accuracy of the numerical model, confirming that the simulator is a reliable tool to predict the motion of a drilling vessel after a blackout.